Dimensionally stable electrodes for anodic reactions in electrolysis cells have recently become of general use in the electrochemical industry replacing the consumable electrodes of carbon, graphite, etc.
Dimensionally stable electrodes generally comprise a valve metal base such as Ti, Ta, Zr, Hf, Nb, and W, which under anodic polarization develop a corrosion-resistant but nonelectrically conductive oxide layer or "barrier layer," coated over at least a portion of their outer surface with an electrically conductive and electrocatalytic layer of platinum group metal oxides or platinum group metals (see U.S. Pat. Nos. 3,711,385; 3,632,498 and 3,846,273). Electroconductive and electrocatalytic coatings made of or containing platinum group metals or platinum group metal oxides are, however, expensive and are eventually subjected to consumption or deactivation in certain electrolytic processes and, therefore, reactivation or recoating is necessary to reactivate exhausted electrodes.
When these electrodes are used in the electrolysis of molten salts, the valve metal support frequently is rapidly dissolved, since the thin protective oxide layer is either not formed at all or is rapidly destroyed by the molten electrolyte with the consequent dissolution of the valve metal base and loss of the noble metal coating.
Numerous patents have taught coatings for various dimensionally stable anodes (see, for example, U.S. Pat. Nos. 4,070,504 and 4,003,817).
Sintered electrodes having electrocatalytic coatings are taught by De Nora in U.S. Pat. No. 4,146,438. He teaches a self-sustaining matrix of sintered powders of an oxycompound of at least one metal selected from a group consisting of 37 metals (including titanium and tantalum) plus the metals of the lanthanide series and the actinide series with at least one electroconductive agent (zirconium oxide and/or tin oxide). He requires that the electrode surface be at least partially coated with at least one electrocatalyst (an oxide of cobalt, nickel, manganese, rhodium, iridium, ruthenium or silver).
Johnson et al. in U.S. Pat. No. 4,160,069 teach a current collector having a ceramic member of rutile which is doped with a polycrystalline ceramic having a valence of at least +5 which has an electrically conductive metal cladding intimately attached to a substantial portion of one surface of the ceramic member.